The present invention relates to a process intended to separate all categories of polymer materials originating from waste.
The invention additionally relates to a plant intended to carry out the process for separating all the categories of polymer materials.
The recycling industry relates to the selective recovery of numerous categories of polymer materials, for example, such as polyethylene (PE), polypropylene (PP), polypropylenes comprising 20 to 40% of talc as filler (PPT), acrylonitrile-butadiene-styrene (ABS), polystyrene (PS), polyamides (PA), poly(methyl methacrylate) (PMMA), poly(vinyl chloride) (PVC), polyurethanes (PU), Xenoy(trademark) (polycarbonate and polyester alloy) and others besides.
The invention relates to a process in stages and to a corresponding plant which ensure the upgrading of spent polymer materials originating from all types of waste and more particularly from motor vehicles. It consequently relates to carrying out a very selective separation of mixtures of polymer materials according to their components and also the conversion of these polymer materials into materials which can be reused or reintroduced in a new manufacturing cycle.
The sorting of polymer materials can be carried out in several ways. Techniques include manual sorting, sorting after cryogenic grinding, sorting with electrostatic processes, sorting by infrared analysis or by laser radiation, density sorting and sorting according to colour and according to shape. Interest will be more particularly shown in the high-throughput sorting of very large volumes of polymer materials of different natures present and mixed among various other materials.
A plant for recycling polymer materials is known from DE-A-4,329,270, this plant comprising in particular a grinder, a carry-over tank for removing the heaviest materials, an air-flow separator for separating the polymers in the form of sheets, and then density separators. On the one hand, in the carry-over tank, polymer materials with a density of greater than 1 can be removed despite their advantages and, on the other hand, there is no cleaning device for polymer materials. This cleaning device proves to be essential in the case of waste originating from public waste dumps or from motor vehicle breakers"" yards.
U.S. Pat. No. 4,728,045 discloses a process for recovering synthetic materials originating from bottles made of polymer materials. A grinding operation, an air-flow separating operation, in order to remove the light materials, such as paper and PP in film form, two separating operations by flotation in a medium with a specific density, in order to separate the PE from the poly(ethylene terephthalate) (PET), and a further aerodynamic separating operation are successively carried out. This process can be applied only to bottles made of polymer materials with a composition of polymer materials precisely determined at the start.
A process for separating polymer materials originating from containers is known from WO-A-92/22 380. This process comprises a grinding stage, an air-flow separating stage, a stage of cleaning the materials and several density separating stages. This process can be applied only to one type of starting substrate with a precisely known composition of polymer materials.
The document Kunststoffberater, 38, June 1993, No. 6, pages 26 to 30, describes a plant comprising a grinder, a washer and density separators. The washer has a rotary-drum device. However, the starting substrate comprises from 95 to 100% of PVC and from 0 to 5% of impurities. This plant is designed only for the separation of PVCs.
The document Kunststoffe, 80, April 1990, No. 4, pages 493 to 495, describes a plant for separating polymer materials, which plant is substantially identical to the preceding plant, comprising a grinder, a cleaning device in the form of a rotary drum with nozzles and density separators.
A process for separating polymer materials is known from FR-A-2,599,279. After grinding and washing, centrifuging takes place in a hydrocyclone, followed by sorting by virtue of a jigging screen.
AT-363,051 presents a process for recovering synthetic materials which consists of a grinding operation, a first flotation operation, a washing operation and a further grinding operation, followed directly by a second flotation operation. These processes give results which are insufficient in terms of quality of the polymer materials obtained after separation.
However, none of these processes cited gives satisfactory results. They are slow or require a very large initial investment. They are also inapplicable to high-throughput sorting and therefore cannot be directly adapted to the industrial scale. Thus, it appears that no separating process of the prior art can be used for sorting mixtures of a wide variety of polymer materials originating from the grinding of motor vehicles or from other sources of waste. Furthermore, they appear to be dedicated to a single or at most to two categories of polymer materials, the other materials being for this reason discharged into the environment.
The problem posed is to provide a process and a corresponding plant for separating complex mixtures of polymer materials of all types which make it possible to separate, purify and obtain all categories of polymer materials. The process and the plant must be effective on an industrial scale and must give a degree of purity of the many different polymer materials similar to the degree of purity of the polymer materials of the first melting process (i.e. initial polymer materials).
The aim of the invention is to overcome the lack of efficiency of the existing techniques by using and by placing, in an appropriate way, an additional mechanical separation stage and two specific phases of density separation in a sequence of cleaning and grinding stages.
According to the invention, the process for separating all categories of polymer materials originating from waste is characterized by the following stages and phases which can be carried out in any order: a grinding stage, a stage of mechanical separation by a shape factor, a cleaning stage and two phases of density separation, in which phases the density is gradually varied in an increasing direction or in a decreasing direction.
These five main stages and phases can be arranged according to several different possibilities. It is possible to begin with a stage of mechanical separation by a shape factor, followed in any order by the other stages or phases envisaged. Alternatively, a grinding stage is followed by a stage of mechanical separation by a shape factor and these two stages are subsequently followed in any order by a stage of cleaning the polymer materials and by two density separation phases. One order has given particularly satisfactory results: a grinding stage is followed by a stage of mechanical separation by a shape factor, which is followed by a first phase of density separation, subsequently followed by a stage of cleaning the polymer materials and finally followed by a second phase of density separation.
In the first phase of density separation, the density is gradually decreased from 1.25 to 1. In the second phase of density separation, the density is gradually increased from 1 to 1.25. The first and/or the second phase of density separation can comprise several stages connected in parallel, the densities then taking discrete values stepwise from one stage to another stage. Alternatively, the first and/or the second phase of density separation can also each be carried out in a single density separator, the density varying continuously according to a smooth or stepwise function. In the latter case, the stages correspond to specific choices of densities, in which choices the choice is made to recover and to remove from the single separator the materials with a density greater than or less than the density value which is specifically chosen and for this reason achieved in the separator. Each of the stages of the first phase is connected in series with each of the stages of the second phase.
The two phases of density separation are preferably carried out by one or more flotation operations in liquid medium. The polymer materials, mixed with other materials, are immersed in a bath. The density of the bath is controlled and adjusted, which makes it possible to separate a material which will float on the surface, with a density lower than that of the liquid medium, from another material which will sink, with a density greater than that of the liquid medium. The materials of interest, which float or which sink, are recovered and are subsequently subjected to further treatments. The density separations can also be carried out by virtue of a mechanical device comprising a densimetric table.
The stage of mechanical separation by a shape factor takes place in particular by virtue of screening means comprising a rotary drum with a calibrated grating which makes it possible to remove all the materials which are too large.
The stage of cleaning the polymer materials makes it possible to restore to the latter a surface which is identical to the surface of materials from a first melting process, that is to say a surface devoid of damaged layers, of paint, of grease, and the like. The cleaning is carried out in liquid medium, preferably with vigorous stirring, at high temperature, in liquid medium with precise compositions.
In order to further refine the separation, to remove the maximum amount of impurities of all sizes and of all materials, and in order to obtain an optimized process, other additional purification stages can be inserted among the main stages. One or more wetting or grinding stages, air-flow separation stages in order to remove the lightest particles or the densest lumps, an electrostatic sorting stage, an optical sorting stage, a stage of separation between two thermoplastics or between thermoplastics and thermosetting plastics by differential melting, a stage of separation by ballistic sorting, a stage of separation by sorting in a fluidized bed, a stage of separation by cryogenic grinding, a stage of density separation by virtue of a mechanical device, rinsing and draining stages, a stage of centrifuging in order to remove any liquid, a drying stage and a stage of storage in a silo are appropriately placed in the line.
In order to obtain separated materials of better quality and so as to improve their respective mechanical properties, the polymers are centrifuged and dried, if wet, then they are homogenized, extruded, centrifuged, graded (xe2x80x9ccompoundedxe2x80x9d) and again homogenized and, finally, the purified polymer materials obtained are bagged up.
By virtue of the invention, the precise sequences of each of the separation stages make it possible to obtain highly purified polymer materials at the end of the process. The various constituent stages or phases of the process can be carried out continuously, stage by stage, or these can be carried out in successive batches with shutdowns and storage for brief periods of time of materials after some of these constituent stages. The latter way, which consists in operating batchwise, allows the process to be perfectly suited to the arrival, in the form of waste, of polymer materials of various categories which are present in variable amounts.
In accordance with the invention, the process makes it possible to obtain PE or extruded PE, PP or extruded PP, PS or extruded PS, ABS or extruded ABS, PPs comprising 20 to 40% of filler, PAs, PMMA, Xenoy(trademark), PVC and others besides.
According to a second aspect of the invention, a plant is characterized in that it comprises the devices for grinding, for density separation with flotation, for mechanical separation by screening, for mechanical separation with a rotary drum, for air-flow separation, for cleaning, for density separation with a mechanical device, for electrostatic separation, for separation by optical sorting, for separation by differential melting, for cryogenic grinding, for separation by ballistic sorting, for separation in a fluidized bed, for wetting, for rinsing and for draining, for centrifuging, for drying, for homogenization, for storage, for extrusion, for grading and for bagging up. These devices follow each other in the plant in the order given by the separation process and by its alternative forms. The preceding devices feed the following devices.